The natural sense of hearing in human beings involves the use of hair cells in the cochlea that convert or transduce acoustic signals into auditory nerve impulses. Hearing loss, which may be due to many different causes, is generally of two types: conductive and sensorineural. Some types of conductive hearing loss occur when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded. These sound pathways may be impeded, for example, by damage to the auditory ossicles. Conductive hearing loss may often be overcome through the use of conventional hearing aids that amplify sound so that acoustic signals can reach the hair cells within the cochlea. Some types of conductive hearing loss may also be treated by surgical procedures.
Sensorineural hearing loss, on the other hand, is caused by the absence or destruction of the hair cells in the cochlea, which are needed to transduce acoustic signals into auditory nerve impulses. People who suffer from severe to profound sensorineural hearing loss may be unable to derive significant benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus. This is because the mechanism for transducing sound energy into auditory nerve impulses has been damaged. Thus, in the absence of properly functioning hair cells, auditory nerve impulses cannot be generated directly from sounds.
To overcome sensorineural hearing loss, numerous cochlear implant systems—or cochlear prostheses—have been developed. Cochlear implant systems bypass the hair cells in the cochlea by presenting electrical stimulation directly to the auditory nerve fibers. Direct stimulation of the auditory nerve fibers leads to the perception of sound in the brain and at least partial restoration of hearing function.
In many people with sensorineural hearing loss, the hair cells deep within the cochlea that sense low frequencies are substantially undamaged. Such people may be able to hear low frequencies without assistance or by means of an amplifying hearing aid. However, due to impaired hearing, the remaining low frequency hearing may be vulnerable to accelerated deterioration. In particular, loss of hearing may increase the stapedial reflex threshold (“SRT”) of a person. The SRT is the loudness at which the brain will cause the stapedial muscle to contract thereby applying tension to the ossicles of the inner ear and reducing the amplitude of vibrations reaching the cochlea by up to 25 dB. As the SRT rises, the low frequency hair cells of the cochlea are exposed to higher intensity vibrations for longer periods of time, thereby accelerating hearing loss and exposing the person to potentially damaging effects caused by sounds with relatively high intensity levels.